Global Positioning System is a navigation and position service offering worldwide, 24 hour coverage. The Global Positioning System (GPS) includes GPS satellites to broadcast GPS satellite signals, control stations to monitor and control the satellites, and a GPS receiver. The GPS receiver demodulates the GPS satellite signals, calculates a pseudorange for each GPS satellite that it receives, and computes a location and a time of observation. A GPS antenna that is a part of the GPS receiver must have a line of sight to a GPS satellite to receive a GPS signal from that satellite.
GPS satellites broadcast two types of location information, P code and C/A code. P-code is intended for use by the United States military and by agencies specifically authorized by the United States military. P-code is encrypted by a Y-code to prevent its unauthorized use. C/A is available to everyone without charge. The C/A code is carried on an L1 carrier at approximately 1.575 GHz. Each GPS satellite modulates C/A code information with a PRN sequence that is unique to the individual GPS satellite. The GPS receiver receives a GPS signal that is a superposition of the GPS satellite signals from all the GPS satellites having a line of sight. The unique PRN sequence enables the GPS receiver to differentiate the GPS signal from each GPS satellite. The GPS receiver computes a pseudorange for each GPS satellite by comparing the time of arrival of the PRN sequence against an internal time standard. The carrier frequency for each GPS satellite signal will vary according to Doppler shift from motion of the GPS satellite and GPS receiver. A description of GPS C/A code is set forth in GPS Interface Control Document ICD-GPS-200, published by Rockwell International Division, Revision A, 26 Sep. 1984, which is incorporated by reference herein.
Commercial GPS receivers are now used for many applications involving timing, navigation, tracking, surveying, and geographical information systems. Some of these applications require that the GPS receiver be carried by an individual user. Typically, where the GPS receiver is carded by an individual user, the user will not have operating access to the power grid or other external power source. Existing GPS receivers intended for handheld or carried applications use batteries for a power source. A limitation of existing GPS receivers intended for carried applications is that the power consumption requires frequent changing or charging of batteries, or large and heavy batteries. Power consumption in existing handheld GPS receivers such as those available from Trimble, Garmin, Sony, Motorola, Panasonic, Rockwell, and Magellan is typically one to two watts. These GPS receivers operate for two to six hours using three to six AA size Alkaline or Nickel Cadmium batteries as recommended by the manufacturers. Existing survey GPS receivers intended to be luggable, such as a Trimble 4000SST, operate for 4 to 8 hours using a battery weighing a few pounds.
Some users extend use time in a GPS receiver by manually cycling power. A limitation of manually cycling power is that users often forget to cycle power off and do not realize their error until the batteries are depleted. GPS receivers are also used or being considered for use by the United States Forest Service to track elk, by marine biologists to track elephant seals (when the seals are on the water surface or on land), and by the FBI to track crime suspects and parolees. A second limitation in these applications is that no user is present to manually cycle power.
A handheld GPS receiver available with a model name Pronav 100 uses six disposable alkaline batteries or a rechargeable battery pack. It also allows the use of an external power source to provide continuous navigation microprocessor updates. The Pronav 100 has a "Battery Saver Mode" operable on six alkaline batteries for fourteen hours with a "QuickFix Mode" which automatically completes four location fixes per hour. Under most dynamic circumstances, use of QuickFix to obtain four position fixes per hour is not satisfactory. The usefulness of the Pronav 100 is limited because the length of battery life is likely to be greatly shortened when the limited operations allowable under the "Battery Saver Mode" or "QuickFix Mode are not sufficient to satisfy the position accuracy requirements.
Another handheld GPS receiver, the Magellan NAV 1000, is powered by six AA alkaline batteries. For the purpose of reducing power consumption and extending the life of the batteries, a "PowerSaveR" mode is provided under which the receiver can be manually turned on to compute a location fix. After the location fix is stored as the last fix, the receiver turns itself off. The receiver can also operate continuously and automatically revert to PowerSaveR mode when a "battery low" condition is detected. The NAV 1000 also allows the units to operate on an external power source. The user's manual includes the instruction not to collect almanac information in handheld operation using the battery because of the concern for battery life. The NAV 1000 is probably less useful due to this limitation. The Trimble Ensign GPS receiver uses automatic on/off power cycling to the RF circuits to provide a mode where location fixes are available at less frequent intervals with less power consumption. However, the maximum off time is less than five seconds which severely limits the power savings.
Typical GPS receivers include the GPS antenna, a GPS engine, one or more user input devices, one or more user output devices, a user microprocessor system to operate the input devices and the output devices, and a power supply. Some existing low power GPS receivers use a Liquid Crystal Display (LCD) for the user output device, and electro-mechanical or touchscreen key switches. Electro-mechanical key switches, where pressing a key completes an electrical connection, are often preferred for low power applications because no power is used except when a key is pressed. An LCD consumes little power in a display but consumes substantial power in a backlight if a backlight is used. Low power GPS receivers having backlit displays generally have a capability to turn off the backlight. The GPS engine includes a GPS frequency downconverter, a Digital Signal Processing (DSP) system having one or more DSP channels, a GPS microprocessor system, and a reference frequency oscillator. Some handheld GPS receivers combine the user microprocessor system and the GPS microprocessor system.
GPS receivers include custom digital and analog circuitry. Most digital circuitry in existing GPS receivers use a generation of components intended to be powered at 5 volts. New generations of GPS engines use components intended to be powered at 3.3 volts. The use of 3.3 volt designs in place of 5 volt designs is expected to reduce the power consumption in the digital circuitry in the GPS receivers by a factor of approximately 2. A second method being used in new generations of GPS receivers to reduce power is higher levels of integration. In general, less power is consumed in a single chip than in two or more chips accomplishing the same function because parasitic capacitances, which require power to charge and discharge, are lower within an integrated circuit than on a printed circuit board or other external wiring system connecting the chips. Existing low power GPS engines use Complementary Metal Oxide Silicon (CMOS) for digital circuitry because CMOS allows high circuit density and consumes less average power than other digital circuit technologies such as Transistor Transistor Logic (TTL), Emitter Coupled Logic (ECL), or N type Metal Oxide Silicon (NMOS). A CMOS circuit consumes power primarily when changing digital states and consumes very little power when the circuit is quiescent. Most digital systems are synchronous where changes in digital states are driven by a clock. Where no clock is present in a synchronous system, the CMOS circuit is quiescent.
Gallium Arsenide (GaAs) and Silicon technologies are used for analog RF circuitry in existing GPS antennas and GPS engines. In either GaAs or Silicon, higher levels of integration reduces power consumption because parasitic capacitances, which require power to charge and discharge, are lower within an integrated circuit than on a printed circuit board or other external wiring system. New GPS receivers are using new RF is designs and fabrication processes to achieve higher levels of integration to reduce power consumption.
Static microprocessors such as the Motorola 68300 series exist that have a low power quiescent mode when not clocked. The microprocessor is placed in the low power quiescent or standby mode in response to executing a "sleep" instruction in software. At least two types of processor sleep instructions are provided to inhibit the clocking of the microprocessor--a "stop" instruction for temporarily disabling a microprocessor clock source and a "wait" instruction for inhibiting a microprocessor clock signal from being passed. The clock signal is made available again by the receipt of a wakeup interrupt signal. Although static microprocessors have been around for a few years, and have been s used in a Trimble "Gamma" GPS receiver, no GPS receiver is known to use the sleep instruction in order to reduce power for a standby mode.
Many U.S. patents disclose power saving methods for signal receivers such as pagers and general communication radios. Basically, a receiver is maintained in a standby state with low power consumption. Either a hardware or a software system is provided to monitor incoming signals. The receiving system is activated when an incoming signal intended for the receiver is detected. The power is automatically turned off after reception of the signals is complete. In some systems, one receiver remains on to reawaken a second receiver. Although the general concept of power saving is widely known, the known methods are not of practical use to a GPS receiver. Unlike the signals received by pagers and communication radios, the GPS satellite signals are intended to be received and used on a continuous basis. Even when not processing GPS satellite signals, GPS receivers are kept busy maintaining tables of visibility, almanacs, ephemeris, and signal strengths for GPS satellites and implementing navigation algorithms. GPS receivers, unlike pagers and communication radios, typically require twenty seconds to three minutes to re-acquire the GPS signal after power is turned on. Unlike pagers and communication radios, where the receiver initiates a request for a response from a user when a signal is received, a GPS user initiates a request for a response from the GPS receiver when the user needs location. Therefore, the methods of maintaining a standby state and passively waiting for the arrival of a signal to save battery power, as disclosed in the U.S. patents for general signal receiving systems, are not useful for reducing power consumption in a GPS receiver.
Some GPS receivers also include dead reckoning devices to enable the GPS receiver to continue to compute and to display new locations when lines of sight to GPS satellite signals are blocked. Existing GPS receivers do not make use of dead reckoning devices to conserve power or to reawaken the GPS receiver after a standby time.
The combination of all existing or known techniques for reducing power consumption in GPS receivers is expected to reduce power by a factor of approximately 3 or 4 to approximately 250 to 750 milliwatts resulting in an increase in battery life to approximately 6 to 24 hours. While users of handheld GPS receivers will appreciate this increase in battery life, a GPS receiver is needed for carried applications that will operate with still lower power for still longer periods of time without recharging or replacing batteries, while providing location fixes at frequent intervals.